[llvm] [AMDGPU] Promote nestedGEP allocas to vectors (PR #141199)
Tim Gymnich via llvm-commits
llvm-commits at lists.llvm.org
Fri May 23 00:23:14 PDT 2025
================
@@ -437,8 +437,62 @@ static Value *GEPToVectorIndex(GetElementPtrInst *GEP, AllocaInst *Alloca,
unsigned BW = DL.getIndexTypeSizeInBits(GEP->getType());
SmallMapVector<Value *, APInt, 4> VarOffsets;
APInt ConstOffset(BW, 0);
- if (GEP->getPointerOperand()->stripPointerCasts() != Alloca ||
- !GEP->collectOffset(DL, BW, VarOffsets, ConstOffset))
+
+ // Walk backwards through nested GEPs to collect both constant and variable
+ // offsets, so that nested vector GEP chains can be lowered in one step.
+ //
+ // Given this IR fragment as input:
+ //
+ // %0 = alloca [10 x <2 x i32>], align 8, addrspace(5)
+ // %1 = getelementptr [10 x <2 x i32>], ptr addrspace(5) %0, i32 0, i32 %j
+ // %2 = getelementptr i8, ptr addrspace(5) %1, i32 4
+ // %3 = load i32, ptr addrspace(5) %2, align 4
+ //
+ // Combine both GEP operations in a single pass, producing:
+ // BasePtr = %0
+ // ConstOffset = 4
+ // VarOffsets = { %j → element_size(<2 x i32>) }
+ //
+ // That lets us emit a single buffer_load directly into a VGPR, without ever
+ // allocating scratch memory for the intermediate pointer.
+ Value *CurPtr = GEP;
+ while (auto *CurGEP = dyn_cast<GetElementPtrInst>(CurPtr)) {
+ SmallMapVector<Value *, APInt, 4> LocalVarsOffsets;
+ APInt LocalConstOffset(BW, 0);
+
+ if (!CurGEP->collectOffset(DL, BW, LocalVarsOffsets, LocalConstOffset))
+ return nullptr;
+
+ // Merge any variable-index contributions into the accumulated VarOffsets
+ // map.
+ // Only a single pointer variable is allowed in the entire GEP chain.
+ // If VarOffsets already holds a different pointer, abort.
+ //
+ // Example:
+ // Suppose LocalVarsOffsets = { (%ptr → 4) } from this GEP, and
+ // VarOffsets already has { (%ptr → 8) } from an inner GEP.
+ // After this loop, VarOffsets should become { (%ptr → 12) }.
+ for (auto &VarEntry : LocalVarsOffsets) {
+ // If VarOffsets already records a different pointer, abort.
+ if (!VarOffsets.empty() && !VarOffsets.count(VarEntry.first))
----------------
tgymnich wrote:
```suggestion
if (!VarOffsets.empty() && !VarOffsets.contains(VarEntry.first))
```
https://github.com/llvm/llvm-project/pull/141199
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